Download L87 THE b PICTORIS MOVING GROUP B. ZUCkERMAN AND

The Astrophysical Journal, 562:L87–L90, 2001 November 20
! 2001. The American Astronomical Society. All rights reserved. Printed in U.S.A.
THE b PICTORIS MOVING GROUP
B. Zuckerman and Inseok Song1
Department of Physics and Astronomy, University of California at Los Angeles, Los Angeles, CA 90095-1562
M. S. Bessell
Research School of Astronomy and Astrophysics, Institute of Advanced Studies, Australian National University,
Private Bag, Weston Creek, Canberra, ACT 2611, Australia
and
R. A. Webb
NASA Ames Research Center, Moffett Field, CA 94035
Received 2001 September 11; accepted 2001 October 9; published 2001 October 30
ABSTRACT
Following the 1983 IRAS detection and subsequent imaging of its extensive dusty circumstellar disk, b Pictoris
became the prototypical and most studied example of a potential forming planetary system. Here we report the
identification of 17 star systems, each with one or more characteristics indicative of extreme youth, that are
moving through space together with b Pic. This diverse set of ∼12 million yr old star systems, which includes
a ∼35 Jupiter mass brown dwarf, and a wide assortment of dusty circumstellar disks, is the comoving, youthful
group closest to Earth. Their unique combination of youth and proximity to Earth makes group members—many
of which have masses similar to that of the Sun—prime candidates for imaging of warm planets and dusty
circumstellar disks with ground- and space-based telescopes.
Subject headings: open clusters and associations: individual (b Pictoris) — stars: kinematics —
stars: pre–main-sequence
et al. (1999) estimated that GJ 799 and GJ 803, and thus
b Pic, are 20 " 10 million yr old.
It is a consensus that most stars, especially relatively massive
ones such as b Pic, form in clusters or associations containing
dozens to hundreds of members. These associations dissociate
with time, but if b Pic is indeed as young as ∼20 Myr, then
we would expect it to have comoving, coeval companions in
addition to GJ 799 and GJ 803. Here we report the identification
of 15 such companion systems (Table 1) where we define a
“system” to contain a star with a Hipparcos catalog number.
Four of these were found as part of our spectroscopic program
at Siding Spring Observatory in New South Wales, Australia.
The others were identified from data in the published literature
sometimes augmented by our supplementary observations. The
total number of stars plus brown dwarfs in Table 1 is 28.
1. INTRODUCTION
The past few years have seen the discovery of numerous
massive extrasolar planets (Marcy, Cochran, & Mayor 2000).
All have been detected indirectly, by virtue of their gravitational
tug on the star about which they orbit. Only when planets are
imaged directly will it be possible to measure their spectra and
thus their compositions. Advances in astronomy from the
ground, specifically, adaptive optics (AO; Beckers 1993), and
the employment of an infrared camera on the Hubble Space
Telescope (HST), now enable imaging detection of planets with
masses comparable to that of Jupiter (Macintosh et al. 2001).
A caveat is that such detections must be of thermal emission
from young, warm planets rather than of reflected starlight from
old, cold planets, such as Jupiter. At wavelengths near a few
microns, thermal emission from a giant planet that is not older
than a few tens of millions of years can be hundreds of times
brighter than reflected starlight; the latter is still much too faint
to be detected with any existing imaging system.
The giant planets of our solar system are 5–30 times more
distant from the Sun than is Earth. Given the diffraction and
instrumental scattered light properties of AO and of HST,
imaging of solar system analogs requires finding stars within
∼40 pc of Earth not older than a few tens of millions of years.
This is a challenging task, notwithstanding tremendous recent
advances in the discovery of “young stars near Earth” (Jayawardhana & Greene 2001); indeed, until recently, few, if any,
such stars had been identified.
An early suggestion that b Pictoris might be as young as
10 Myr (Jura et al. 1993) was “hard to swallow” because of
the apparent isolation of b Pic in space. Then, Barrado y Navascués et al. (1999) found that the low-mass star systems GJ
799 and GJ 803 are moving through space together with b Pic.
Hence, all three stars likely formed in proximity and at nearly
the same time. Using standard techniques, Barrado y Navascués
1
2. OBSERVATIONS
At Siding Spring, we are in the midst of an extensive survey
to identify young stars near Earth (I. Song, M. S. Bessell, & B.
Zuckerman 2001, in preparation). We are using the double-beam
grating spectrograph (DBS) and the echelle spectrograph on the
two Nasmyth foci of the Australian National University’s 2.3 m
telescope. The red channel of the DBS covered the spectral range 6500–7450 Å at a measured resolution of 1.16 Å. The
blue-beam data are discussed in I. Song et al. (2001, in preparation).
Eight orders of the echelle covered portions of the spectra
between 5800 and 7230 Å but with rather large gaps between
the orders. We focused our attention on orders that contained
the Ha and lithium 6708 Å lines. In these orders, the measured
resolution was 0.40 Å. Radial velocities were determined by
cross-correlating target and standard spectra over five or six spectral orders. Projected rotational velocities (v sin i ) were measured
from a total of about 10 lines in the Li 6708 Å echelle order
with a procedure similar to that described in Strassmeier et al.
(1990). The DBS and echelle spectra, obtained in various ob-
UCLA Center for Astrobiology postdoctoral fellow.
L87
L88
b PICTORIS MOVING GROUP
Vol. 562
TABLE 1
The b Pictoris Moving Group
HIP
Other Name
560 . . . . . . . . . .
21547 . . . . . . .
HR 9
51 Eri
GJ 3305
23309
25486
27321
29964
76629
79881
84586
.......
.......
.......
.......
.......
.......
.......
88399 . . . . . . .
88726 . . . . . . .
92024 . . . . . . .
92680 . . . . . . .
95261 . . . . . . .
95270 . . . . . . .
102141 . . . . . .
102409 . . . . . .
103311 . . . . . .
HD 35850
b Pic
AO Men
V343 Nor
HR 6070
HD 155555A
HD 155555B
HD 155555C
HD 164249
HR 6749
HD 172555
CD !64#1208
PZ Tel
HR 7329
HR 7329B
HD 181327
GJ 799
GJ 803
HD 199143
BD !17#6128
R.A.
00
04
04
05
05
05
06
15
16
17
17
17
18
18
18
18
18
19
19
19
20
20
20
20
06
37
37
00
27
47
18
38
18
17
17
17
03
06
45
45
53
22
22
22
41
45
55
56
50.0
36.1
37.3
47.1
04.7
17.1
28.2
57.6
17.9
25.5
25.5
27.7
03.4
49.9
26.9
36.9
05.9
51.2
51.3
58.9
51.0
09.3
47.7
02.7
Decl.
!23
!02
!02
!57
!11
!51
!72
!57
!28
!66
!66
!66
!51
!43
!64
!64
!50
!54
!54
!54
!32
!31
!17
!17
06
28
29
15
54
04
02
42
36
57
57
57
38
25
52
51
10
25
25
32
26
20
06
10
27
24
37
26
03
00
42
27
50
02
02
00
56
30
15
48
49
25
29
16
04
24
51
54
Spectral
Type
Distance
(pc)
V
(mag)
V!ICa
F2 IV
F0 V
M0.5
M0/1
F7
A3 V
K6/7
K0 V
A0
G5 IV
K0 IV/V
M4.5
F5 V
A5 V
A7
K7
K0 Vp
A0 Vn
M7/8 V
F5.5
M4.5e
M1e
F8 V
K7/M0
39.1
29.8
29.8
26.3
26.8
19.3
38.5
39.8
43.0
31.4
31.4
31.4
46.9
43.9
29.2
29.2
49.7
47.7
47.7
50.6
10.2
9.9
47.7
47.7
6.19
5.22
10.59
10.01
6.30
3.85
9.77
8.14
4.80
7.21
8.08
12.71
7.01
5.67
4.78
9.54
8.25
5.05
11.93e
7.03
11.02
8.81
7.34
10.6
0.461
0.342
1.90
1.79
0.616
0.199
1.34
0.93
!0.01
0.81
1.09
2.69
0.53
0.26
0.204
1.545
0.85
0.04
…
0.53
2.90
2.10
0.62
…
v sin i
(km s!1)
LX/Lbol b
(#10!3)
(U, V, W)c
(km s!1)
155
95
…
11
50
139
13
11
30
37
34
…
20
…
134
0.005
…
3.2
0.3
0.3
…
1.3
0.6
…
0.95
0.95
0.95
0.33
…
0.002
0.86
0.77
…
…
…
1.3
1.4
0.45
0.36
!11.2, !15.0, !10.1
!13.9, !16.0, !10.3
…
!10.9, !15.4, !8.1
!10.3, !15.1, !8.0
!10.8, !16.4, !8.9
!10.3, !15.2, !8.2
!11.2, !15.7, !9.4
!13.6, !16.8, !12.4
!10.6, !15.1, !8.2
…
…
!7.3, !15.4, !8.8
!11.4, !17.3, !10.0
!10.9, !15.8, !8.8
…
!7.7, !16.6, !9.2
!10.8, !14.7, !7.7
…
!10.1, !16.0, !8.8
!9.6, !16.8, !11.1
!10.5, !16.6, !10.2
!9.1, !14.7, !10.6
!9.0, !15.0, !10.8
kd
63
kd
…
16
…
9
120
12
Notes
1
1, 2
2
3
4
1, 3
3
3
1
5
5
5
3, 6
1, 7
1, 8
9
1, 10
10
7, 11
11, 12
13
13
Notes.—Units of right ascension are hours, minutes, and seconds, and units of declination are degrees, arcminutes, and arcseconds. (1) On A star zero-age
main sequence (Jura et al. 1998). (2) Binary: listed ROSAT flux and LX/Lbol are for the secondary. (3) See Table 2 for additional details. (4) See Tagliaferri et al.
1997, Mathioudakis & Mullan 1999, and Gagné et al. 1999 for additional details. See Silverstone 2000 for Infrared Space Observatory far-IR excess flux.
(5) ROSAT X-ray flux (and thus LX/Lbol) is for total of A " B " C; see Pasquini et al. 1991, Martin & Brandner 1995, Hatzes & Kürster 1999, and Strassmeier
& Rice 2000 for additional details. (6) See Silverstone 2000 for IRAS excess flux. (7) Equal brightness binary; mv is for each individual component. (8) See Coté
1987 and Mannings & Barlow 1998 for IRAS excess fluxes. (9) Spectral type is estimated from BVRC IC photometry (B. Shobbrook 2001, private communication);
V p 9.535, B!V p 1.189, V!RC p 0.757, and V!IC p 1.545. A spectrum obtained by M. Buxton (2001, private communication) with the DBS shows strong
Ha in emission (EW ∼ 2.2 Å), a strong Li 6708 Å feature (EW ∼ 580 mÅ), and fast rotation. The unusually strong Li for an ∼10 Myr old star may be due to
the very fast rotation (Soderblom et al. 1993). (10) See Mannings & Barlow 1998 for IRAS excess fluxes. (11) See Barrado y Navascués et al. 1999 for additional
details. (12) One of only two M-type stars not near a molecular cloud with IRAS-detected excess infrared emission (Song et al. 2001). (13) HD 199143 and BD
!17#6128 are themselves both close unequal brightness binaries (D. Kaisler et al. 2001, in preparation), so this is a quadruple system; mv is for the total of the
primary and secondary at each named star.
a
V!IC data are from many different sources (see I. Song, M. S. Bessell, & B. Zuckerman 2001, in preparation, for details).
b
Ratio of X-ray luminosity (based on the ROSAT All-Sky Survey X-ray count rate) to total stellar luminosity.
c
Based on averaged proper motions from PPM and Hipparcos catalogs. UVW are positive in the directions of the Galactic center, Galactic rotation, and the
north Galactic pole. Errors in UVW are all smaller than 2.0 km s!1 in all components except for HIP 88399, 88726, and 92024, where poor radial velocity
measurements result in errors, mainly in the U-component, of 3–5 km s!1.
d
v sin i ! 100 km s!1.
e
H magnitude.
serving runs between 2001 January and June, were reduced and
calibrated in IRAF. Additional details can be found in I. Song
et al. (2001, in preparation).
With the echelle spectrograph, we measure stellar radial velocities, which, combined with proper motions and distances
from the Hipparcos and Positions and Proper Motions (PPM)
catalogs, yield Galactic three-dimensional space motions
(UVW) with respect to the Sun. These measurements indicate
that HIP 23309, 29964, 76629, and 88399 are comoving
through space with b Pic (see Tables 1 and 2).
3. DISCUSSION
We considered ∼22,000 stars whose space motion can be
calculated from data that exist in the literature, specifically,
stars with measured radial velocities and with Hipparcos parallaxes. We searched for stars within 50 pc of Earth whose
UVW velocity components are each within a few kilometers
per second of those of b Pic. We further winnowed down this
list by demanding that a star have at least one strong signpost
of extreme youth. For stars listed in Table 1, these signs include
(1) a large LX/Lbol for GJ 3305, HIP 23309, 25486, 29964,
76629, 84586, 88399, 92680, 102141, 102409, and 103311,
BD !17#6128, and CD !64#1208; (2) large v sin i (rapid rotation) for HIP 23309, 25486, 29964, 76629, 84586, 88399,
92680, 102409, and 103311, BD !17#6128, and CD
!64#1208; (3) large lithium abundance for HIP 23309, 29964,
76629, 92680, and 102409, BD !17#6128, and CD !64#1208;
(4) Ha in emission or filled in for HIP 23309, 29964, 92680,
102141, and 102409; (5) An A or early F dwarf on the zeroage main sequence for HIP 560, 21547, 27321, 79881, 88726,
92024, and 95261; (6) optical variability for HIP 29964, 76629,
84586, 92680, 102141, 102409, 103311, and BD !17#6128;
(7) excess mid- or far-IR emission for HIP 25486, 27321,
88399, 92024, 95261, 95270, and 102409; and (8) for lowmass stars, a location well above the main sequence on a colormagnitude diagram (Fig. 1).
All 18 star systems that survived this procedure—“the b Pic
moving group”—lie in the southern hemisphere (Table 1), notwithstanding that most of the 22,000 stars are located in the
northern hemisphere. The average distance from Earth of the
Table 1 systems is ∼35 pc. Because of the 50 pc cutoff in our
sample selection and the fact that all Table 1 stars lie on one
side of the sky, it is possible that members of the moving group
remain to be identified at southern declinations beyond 50 pc.
No. 1, 2001
ZUCKERMAN ET AL.
L89
TABLE 2
Measured and Derived Quantitiesa
Quantity
mR.A. (mas yr!1):
PPM . . . . . . . . . . . . . . . . . . . . . . . .
Hipparcos . . . . . . . . . . . . . . . . . . .
mdecl. (mas yr!1):
PPM . . . . . . . . . . . . . . . . . . . . . . . .
Hipparcos . . . . . . . . . . . . . . . . . . .
Radial velocity (km s!1) . . . . . .
Teff (K) . . . . . . . . . . . . . . . . . . . . . . . . .
B!V . . . . . . . . . . . . . . . . . . . . . . . . . . .
V ...............................
V!IC . . . . . . . . . . . . . . . . . . . . . . . . . . .
J ...............................
H ...............................
K ...............................
Li 6708 EW (mÅ)c . . . . . . . . . . .
Ha EW (Å)d . . . . . . . . . . . . . . . . . .
ROSAT (counts s!1) . . . . . . . . . . .
HIP 23309
35
36
76
73
17.8 " 0.8
3810
1.421
10.01
1.79
…
…
…
294
!0.81
0.33
HIP 29964b
(HD 45081)
!11
!8
HIP 76629
(HD 139084)
!51
!53
75
71
15.0 " 1.0
4288
1.11
9.77
1.34
7.55
6.99
6.81
357
!0.65
1.03
!97
!106
0.5 " 0.9
5250
0.82
8.06
0.93
…
…
…
261
0.49
1.42
HIP 88399
(HD 164249)
b Pic
(HIP 27321)
6.6
3.5
9.4
4.7
!93
!86
0.1 " 3.0
6420
0.458
7.01
…
…
…
…
92
1.46
0.15
79
82
20.2 " 0.4
8500
0.171
3.85
0.18
3.55
3.47
3.49
…
!
…
a
Radial velocity, Li, and Ha EWs for HIP 23309, 29964, and 76629 are from our echelle spectra; B!V, V,
V!IC for HIP 23309 and 29964 are from L. Berdnikov (2001, private communication); J, H, and K for HIP
29964 are from the Two Micron All Sky Survey catalog; radial velocity for b Pic is from Barrado y Navascués
et al. 1999. Colors and V for HIP 76629 are from Cutispoto et al. 1999.
b
Cutispoto et al. 1999 list a set of measurements for HIP 29964; radial velocity p 16.2 " 0.7 km s!1,
v sin i p 17 " 2 km s!1, (B!V) p 1.13, V p 9.80, and (V!IC) p 1.32. Average radial velocity of our value,
15.0 km s!1, and Cutispoto et al.’s was used to calculate (U, V, W).
c
Measured lithium equivalent widths corrected for the contribution of an Fe i l6707.44 line. Corrections are
25, 19, 13, and 6 mÅ for HIP 23309, 29964, 76629, and 88399, respectively (Soderblom et al. 1993).
d
Line core equivalent widths of Ha. The minus sign indicates emission.
Other known kinematically related associations of nearby
young stars include Tucana/Horologium at 45 pc (Zuckerman
& Webb 2000; Torres et al. 2000; Stelzer & Neuhäuser 2000;
Zuckerman, Song, & Webb 2001), TW Hydrae at 60 pc (Kastner et al. 1997; Webb et al. 1999), and h Chamaeleontis at
97 pc (Mamajek, Lawson, & Feigelson 1999). These associations, which also are located in the southern hemisphere, are
distinguishable by their differing age, space motions, and locations in space. Indeed, based on the V and W components
of space motion and age diagnostics, we include in Table 1
Fig. 1.—H-R diagram showing 3, 10, and 30 million yr solar metallicity
isochrones from Siess, Dufour, & Forestini (2000), Palla & Stahler (1993),
and D’Antona & Mazzitelli (1997). Measured quantities listed in Table 1 (V,
V!IC, and distance from Earth) were translated to luminosity (L) and effective
temperature (Teff) with relationships corresponding to the 12 Myr isochrone in
Siess et al. (2000) with color-to-temperature conversions of Siess, Forestini,
& Dougados (1997). For BD !17#6128, L p 0.34 " 0.06 L, and Teff p
3960 K were used (van den Ancker et al. 2001). The thick shaded line represents the zero-age main sequence from Siess et al. (2000).
three systems that were originally suggested to be members of
the Tucana stream (HIP 92680, 95261, and 95270; Zuckerman
& Webb 2000). HIP 95261 (HR 7329) has a brown dwarf
companion of probable age ∼10 Myr and mass ∼35 times that
of Jupiter (Lowrance et al. 2000; Guenther et al. 2001).
The associations listed above are of interest for various reasons. But when one’s focus is the study of planets and circumstellar disks, the stars in the b Pic moving group have
important advantages over all others because Table 1 stars are
substantially closer than stars in TW Hydrae and h Chamaeleontis, and substantially younger than stars in Tucana/Horologium (see discussion below).
Not included in the above list of nearby very young associations is the unusual Capricornus group at 48 pc (van den Ancker
et al. 2000) because we have subsumed it into the b Pic moving
group. This resulted from our accurate measurement of the radial
velocity of BD !17#6128 (Vrad p !6.3 " 0.9 km s!1; D. Kaisler et al. 2001, in preparation), which enabled us to calculate
UVW for it and for HD 199143. In addition, with adaptive optics
at the Lick Observatory 3 m telescope, we resolved both BD
!17#6128 and HD 199143 as 1"–2" separation binaries (D. Kaisler et al. 2001, in preparation).
To be listed in Table 1, in addition to having the same space
motion as b Pic, a star must display at least one clear sign of
extreme youth because some older and unrelated stars may accidentally have this same space motion. We estimated the expected number of such (old) stars as follows. From the set of
∼22,000 Hipparcos stars with radial velocities known from the
literature, we found 497 stars within 50 pc of Earth with
!30 ! U ! 0, !30 ! V ! 0, and !20 ! W ! 0. This volume [in
(km s!1) 3] of UVW space is densely and fairly uniformly filled
with the 497 stars and well encompasses the UVW range of the
stars in Table 1. The UVW volume of Table 1 stars is 81
(km s!1)3. Thus, we anticipate that two old stars would lie in
the UVW volume of Table 1 stars, but none appear among the
L90
b PICTORIS MOVING GROUP
sample of 497. In addition, those few stars within 50 pc of Earth
with UVW slightly outside of the volume of velocity space defined by Table 1 stars show no obvious signs of extreme youth.
The velocity range, ≤"2 km s!1, displayed by Table 1 stars
is characteristic of velocity dispersions measured in star-forming
interstellar molecular clouds. Thus, plausibly, the b Pic group
formed in such a cloud or cloud complex. Some of the velocity
dispersion apparent in UVW may be due to measurement error,
especially in radial velocity. For most of the Table 1 stars, radial
velocity errors are ≤2 km s!1. For the A and early F-type stars
and for rapidly rotating lower mass stars, radial velocity errors
can be twice as large.
The large spatial extent of the b Pic moving group (at its
young age) implies that it never formed a tightly bound cluster
like the Pleiades. This is also apparent from the existence of
many wide binaries (∼3000 AU) and the large velocity dispersion of Table 1 stars compared to Pleiades stars (velocity
dispersion of 0.42 km s!1 per axis; Jones 1970).
The age of the b Pic moving group may be derived from
the properties of its lower mass members, those of spectral type
late K and M. Ages of young, low-mass stars may be estimated
in two ways from theoretical models. One technique involves
placement of a star on pre–main-sequence evolutionary tracks
in a Hertzsprung-Russell (H-R) diagram; the other technique
is based on the time required to deplete lithium in a stellar
atmosphere. Based on the theoretical models, ages of ∼30 Myr
have been estimated for stars in the Tucana and Horologium
associations and ∼10 Myr for members of the TW Hydrae
association. From the location on the H-R diagram (Fig. 1),
other age indicators as listed above, and lithium equivalent
widths intermediate in strength between Tucana-Horologium
and TW Hydrae stars, we estimate that the b Pic moving group
is 12"8
!4 Myr old. Thus, we define the group as nearby stars that
have UVW velocity each within ∼2 km s!1 of those of b Pic
and age consistent with 12 Myr.
Vol. 562
Some stars in Table 1 have been extensively studied appearing in hundreds of papers listed in SIMBAD. At the opposite extreme, HIP 23309 is so obscure that, as of this writing,
there are zero references to it in SIMBAD, despite its station
as the star in Table 1 closest to b Pic (separation 8 pc). In any
event, the existing literature portrays the stars in Table 1 as a
hodgepodge of uncertain age and overall status; recognition as
members of a common group of known age brings welcome
order to this wonderfully diverse stellar collection.
4. CONCLUSION
The b Pic moving group contains many of the youngest,
closest stars to Earth. All systems listed in Table 1 include a
Hipparcos star so that parallaxes are known and, thus, UVW
space motions can be calculated. In our program in progress
at Siding Spring and Lick observatories, we have spectroscopically identified young, nearby Tycho Catalog stars that—
pending measurements of accurate parallaxes so that UVW can
be determined—may also be b Pic group members (I. Song,
M. S. Bessell, & B. Zuckerman 2001, in preparation). As new
ground- and space-based techniques and instruments are developed during the coming decades, surely members of the
b Pic moving group will be prime targets in the investigation
of forming planetary systems.
We thank Leonid Berdnikov for measuring the BVIC magnitudes of HIP 23309 and 29964, Bob Shobbrook for measuring
BVRCIC magnitudes for CD !64#1208, Michelle Buxton for
measuring the spectrum of CD !64#1208, and Denise Kaisler
for early assistance with calculations of UVW velocities. We
appreciate helpful comments from Eric Becklin and referee
John Stauffer. This research was supported in part by the UCLA
Astrobiology Institute and by a NASA grant to UCLA.
REFERENCES
Barrado y Navascués, D., Stauffer, J. R., Song, I., & Caillault, J.-P. 1999, ApJ,
520, L123
Beckers, J. M. 1993, ARA&A, 31, 13
Coté, J. 1987, A&A, 181, 77
Cutispoto, G., Pastori, L., Tagliaferri, G., Messina, S., & Pallavicini, R. 1999,
A&AS, 138, 87
D’Antona, R. A., & Mazzitelli, I. 1997, Mem. Soc. Astron. Italiana, 68, 807
Gagné, M., Valenti, J. A., Linsky, J. L., Tagliaferri, G., Covino, S., & Güdel,
M. 1999, ApJ, 515, 423
Guenther, E. W., Neuhäuser, R., Huélamo, N., Brandner, W., & Alves, J. 2001,
A&A, 365, 514
Hatzes, A. P., & Kürster, M. 1999, A&A, 346, 432
Jayawardhana, R., & Greene, T., eds. 2001, ASP Conf. Ser. 244, Young Stars
near Earth: Progress and Prospects (San Francisco: ASP)
Jones, B. 1970, AJ, 75, 563
Jura, M., Malkan, M., White, R., Telesco, C., Pina, R., & Fisher, R. S. 1998,
ApJ, 505, 897
Jura, M., Zuckerman, B., Becklin, E. E., & Smith, R. C. 1993, ApJ, 418, L37
Kastner, J. H., Zuckerman, B., Weintraub, D. A., & Forveille, T. 1997, Science,
277, 67
Lowrance, P. J., et al. 2000, ApJ, 541, 390
Macintosh, B., et al. 2001, in ASP Conf. Ser. 244, Young Stars near Earth:
Progress and Prospects, ed. R. Jayawardhana & T. Greene (San Francisco:
ASP), 309
Mamajek, E. E., Lawson, W. A., & Feigelson, E. D. 1999, ApJ, 516, L77
Mannings, V., & Barlow, M. J. 1998, ApJ, 497, 330
Marcy, G. W., Cochran, W. D., & Mayor, M. 2000, in Protostars and Planets
IV, ed. V. Mannings, A. P. Boss, & S. S. Russell (Tucson: Univ. Arizona
Press), 1285
Martin, E. L., & Brandner, W. 1995, A&A, 294, 744
Mathioudakis, M., & Mullan, D. J. 1999, A&A, 342, 524
Palla, F., & Stahler, S. W. 1993, ApJ, 418, 414
Pasquini, L., Cutispoto, G., Gratton, R., & Mayor, M. 1991, A&A, 248, 72
Siess, L., Dufour, E., & Forestini, M. 2000, A&A, 358, 593
Siess, L., Forestini, M., & Dougados, C. 1997, A&A, 324, 556
Silverstone, M. D. 2000, Ph.D. thesis, Univ. California, Los Angeles
Soderblom, D. R., Jones, B. F., Balachandran, S., Stauffer, J. R., Duncan,
D. K., Fedele, S. B., & Hudon, J. D. 1993, AJ, 106, 1059
Song, I., Weinberger, A., Becklin, E. E., Zuckerman, B., & Chen, C. 2001,
AJ, in press
Stelzer, B., & Neuhäuser, R. 2000, A&A, 361, 581
Strassmeier, K. G., Fekel, F. C., Bopp, B. W., Dempsey, R. C., & Henry,
G. W. 1990, ApJS, 72, 191
Strassmeier, K. G., & Rice, J. B. 2000, A&A, 360, 1019
Tagliaferri, G., Covino, S., Fleming, T. A., Gagne, M., Pallavicini, R., Haardt,
F., & Uchida, Y. 1997, A&A, 321, 850
Torres, C. A. O., da Silva, L., Quast, G. R., de la Reza, R., & Jilinski, E.
2000, AJ, 120, 1410
van den Ancker, M. E., Pérez, M. R., de Winter, D., & McCollum, B. 2000,
A&A, 363, L25
van den Ancker, M. E., et al. 2001, in ASP Conf. Ser. 244, Young Stars near
Earth: Progress and Prospects, ed. R. Jayawardhana & T. Greene (San Francisco: ASP), 69
Webb, R. A., Zuckerman, B., Platais, I., Patience, J., White, R. J., Schwartz,
M. J., & McCarthy, C. 1999, ApJ, 512, L63
Zuckerman, B., Song, I., & Webb, R. A. 2001, ApJ, 559, 388
Zuckerman, B., & Webb, R. A. 2000, ApJ, 535, 959